Method and apparatus for testing coins

ABSTRACT

A method and apparatus for testing coins is described. In particular, the resistance introduced into a tuned circuit by the proximity of a coin while it is moving past an inductor of the circuit is determined by changing the amount of phase shift present in a feedback path associated with the circuit and measuring the resulting change in frequency of oscillation, which is dependent upon the resistance in the tune circuit.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for testing coins.

BACKGROUND OF THE INVENTION

In this specification, the term "coin" is used to encompass genuinecoins, tokens, counterfeit coins and any other objects which may be usedin an attempt to operate coin-operated equipment.

Coin testing apparatus is well known in which a coin is subjected to atest by passing it through a passageway in which it enters anoscillating magnetic field produced by an inductor and measuring thedegree of interaction between the coin and the field, the resultingmeasurement being dependent upon one or more characteristics of the coinand being compared with a reference value, or each of a set of referencevalues, corresponding to the measurement obtained from one or moredenominations of acceptable coin. It is most usual to apply more thanone such test, the respective tests being responsive to respectivedifferent coin characteristics, and to judge the tested coin acceptableonly if all the test results are appropriate to a single, acceptable,denomination of coin. An example of such apparatus is described inGB-A-2 093 620.

One particular test which is often applied is to determine the maximumeffect that the coin has on the amplitude of a signal derived from theinductor. This may be done simply by measuring the peak value that theamplitude reaches as the coin passes by the inductor, or measuring boththat peak amplitude, and also the amplitude when the coin is notadjacent to the inductor and taking a function of (for example, eitherthe difference between, or the ratio of) those two amplitudes so as toobtain a value which is less influenced by drift in the circuitry andvariations in component parameters. These tests based on amplitude givean indication of the effective resistance (or loss) that is introducedinto the inductor circuit by the coin when the coin is sufficientlyclose to the inductor that eddy currents are being induced in it.

In EP-B1-0 062 411 there is disclosed a method of testing coins inwhich, as one feature, the effective resistance or loss of a coil, asinfluenced by a coin held stationary adjacent the coil, is measured byswitching a phase change repeatedly into, and out of, the feed back loopof an oscillating tuned circuit, measuring the oscillation frequencywith the phase change in the circuit, and without the phase change inthe circuit, and taking the difference between the two measuredfrequencies as an indication of effective resistance. It is inherent inthat method that frequency measurements have to be taken on the samecoin, using the same circuit, but at different times. To enable this tobe done, EP-B1-0 062 411 proposes that after the arrival of a coin inthe testing apparatus has been detected a delay of one third of a secondis provided to allow the coin to come to rest in a fixed stable positionagainst a stop in a coin runway, where the coin is located between thetwo halves of a testing coil. When the coin is in that fixed position,the phase change is repeatedly switched into and out of the oscillatorcircuit for periods which are at least 3.75 ms long, and this is donemany times whilst frequency measurements are taken, the coin then beingreleased by the stop to continue its passage through the testingapparatus.

Although in principle this is a useful way of measuring resistance orloss, in practice the need to hold the coin stationary makes the methodand apparatus unsuitable for testing a succession of coins rapidly oneafter the other, which is a requirement in most practical applicationsof coin testing apparatus.

SUMMARY OF THE INVENTION

The invention involves the realisation that, contrary to the disclosurein the above prior art, it is possible to perform a similar method ofmeasuring effective resistance or loss while the coin is actually movingpast the inductor of a tuned circuit.

More particularly, the invention provides a method of testing coinsusing an oscillating tuned circuit which includes an inductor, threeparameters of the tuned circuit being interdependent, namely:

a) the effective resistance in the circuit

b) the phase of a signal in the circuit, and

c) the frequency of oscillation of the circuit, the method comprisingimposing a change in said phase when a coin is adjacent to the inductor,deriving from the resulting frequency change a value dependent on theeffective resistance in the tuned circuit as influenced by the coin, andusing the derived value in a coin acceptability check,

characterised by causing the coin to move past the inductor during saidphase change and the resulting frequency change.

If, in relation to the speed of the coin, the time interval betweenmeasuring the frequency with phase change and measuring it without phasechange is made sufficiently short, the change in coin position occurringbetween the two measurements does not introduce an error in theeffective resistance measurement sufficiently great to render theresults unacceptably inaccurate.

However, preferably, the derived value, which is dependent on theeffective resistance in the tuned circuit as influence by the coin, iscompensated for the effect of the change in position of the moving coinoccurring between the two frequency measurements. In this way theaccuracy of the measurement can be improved, or a higher coin speed canbe accommodated, or a lower phase change switching rate can be employed.This is especially the case when the measurements to be used for coinvalidation are taken at a time when the oscillation frequency ischanging, and especially when it is changing quickly, due to themovement of the coin.

Preferably, the method comprises repeatedly imposing, then removing,said phase change, repeatedly measuring said frequency with and withoutthe imposed phase change, interpolating between either the frequencyvalues measured with the phase change, or those measured without thephase change, to develop compensated frequency values, and utilising thecompensated frequency values in deriving said resistance-dependantvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, twoembodiments will now be described, by way of example, with reference tothe accompanying diagrammatic drawings in which:

FIG. 1 shows schematically a first embodiment of the invention,

FIG. 2 illustrates the relationship between frequency, phase andeffective resistance in a tuned circuit, and

FIG. 3 illustrates how the embodiment of FIG. 1 may be modified toenable compensation for coin movement to be applied.

DETAILED DESCRIPTION

Referring to FIG. 1, a pi-configuration tuned circuit 2 includes aninductor in the form of a single coil 4, two capacitors 6 and 7 and aresistor 8. Resistor 8 is not normally a separate component and shouldbe regarded as representing the effective resistance in the tunedcircuit, which will consist primarily of the inherent resistance of thecoil 4.

Means is provided for moving a coin shown in broken lines at 10 past andadjacent to the coil 4, the means being shown schematically as a coinpassageway 12 along which the coin moves on edge past the coil. Apractical arrangement for passing a moving coin adjacent to an inductivetesting coil is shown, for example, in GB-A-2 093 620, the disclosure ofwhich is incorporated herein by reference. As the coin 10 moves past thecoil 4, the total effective resistance in the tuned circuit increases,reaching a peak when the coin is centred relative to the coil, and thendecreases to an idling level. In the present example the apparatus isresponsive to the peak value of this effective resistance.

The tuned circuit 2 is provided with a feedback path so as to form afree-running oscillator. The feedback path is generally indicated at 14and includes a line 16 which carries the voltage occurring at one pointin the tuned circuit, a switching circuit 18, and an inverting amplifier20 which provides gain in the feedback path. A phase delay circuit shownschematically at 24 is alternately switched into the feedback path, orby-passed, depending on the condition of switching circuit 18. The phaseshift round the feedback path is 180° when the phase delay circuit 24 isnot switched into it, and the phase shift across the pi-configurationtuned circuit is then also 180°. In this condition the oscillator runsat its resonant frequency.

It is convenient now to refer to FIG. 2. FIG. 2 shows the relationshipbetween frequency of oscillation and amount of phase shift (φ) in thefeedback path for five different values of total effective resistance inthe tuned circuit, from a relatively low value R1 to a relatively highvalue R5. In general terms, for a pi-configuration tuned circuit inwhich the effective resistance is variable, the amount of effectiveresistance in the circuit at any particular time can be determined bychanging the amount of phase shift in the feedback path from one knownvalue to another (or by a known amount) and measuring the resultingchange in frequency. The relationship between the phase shift change andthe frequency change effectively represents the gradient of one of thecurves shown in FIG. 2 and consequently indicates on which curve thecircuit is operating and hence what is the present effective resistancein the circuit. For example, if the phase shift is changed from 180° byan amount φ1 (which may be about 30°) as shown and the frequency changesby ΔfNC then the effective resistance is the low value R1; but, if thefrequency changes by the larger amount ΔfC the effective resistance isthe higher value R4.

This technique is implemented for testing coins by the circuitryschematically shown in FIG. 1, the description of which will now becompleted.

The frequency of the oscillator is fed on line 26 to a frequency sensingcircuit 28. A control circuit 30 repeatedly operates switching circuit18 by a line 32 to switch the phase delay circuit 24 into and out of theoscillator feedback path. Via the same line 32 it also operates a switch34 in synchronism with switching circuit 18 so that the values of thefrequency sensed by sensing circuit 28 are stored in store 36 (thisbeing the frequency value when the phase delay is not present in theoscillator circuit) and store 38 (this being the frequency value whenthe phase delay is introduced into the oscillator circuit). FIG. 1 andthe following description may be better understood by reference to thefollowing table of the notation used for various frequencies andfrequency differences:

f0=frequency without phase shift

fφ=frequency with phase shift

Δf=fφ-f0

ΔfNC=Δf when coin absent

ΔfC=peak value of Δf when coin present

fOC=peak value of f0 when coin present

fONC=value of f0 when coin absent

A subtracter 40 subtracts f0 from fφ to develop Δf and, in the normalcondition of a switch 42, this value of Δf is passed to a store 44. Thisnormal condition prevails while there is no coin adjacent to coil 4, inwhich case the effective resistance in the tuned circuit is low (say,the low value R1 of FIG. 2) and the frequency difference value beingstored at 44 is then ΔfNC (indicated in FIG. 2), this value beingindicative of the inherent effective resistance of the tuned circuititself at the time when the measurements are being taken.

As a coin 10 begins to arrive adjacent to coil 4, f0 at the output offrequency sensing circuit 28 starts to change. A section 46 of controlcircuit 30 detects the beginning of this change from line 48 and inresponse changes the condition of switch 42 via line 50, causing therecent idling value of ΔfNC to be held in store 44.

As the coin 10 approaches and reaches a position central relative tocoil 4, so the frequency f0 falls until it reaches a peak low value.Circuit section 46 is adapted to detect this peak occurring and, inresponse, it causes switch 42 to direct the value of Δf occurring whenthe coin is centred, to store 52. This is value ΔfC, for example, asshown on FIG. 2, and it is the maximum value of frequency shiftresulting from the imposed phase change φ1 that occurs during thepassage of the coin past the inductor. This frequency shift indicatesthat the total effective resistance in the tuned circuit is now therelatively high value R4 consisting of the effective resistance inherentin the circuit plus the effective resistance introduced into it by theparticular coin which is now centred on the coil 4. A value indicativeof the effective resistance introduced by the coin alone is then derivedby subtracter 54 which subtracts ΔfNC from ΔfC.

The resulting signal is compared in a comparison stage 56 with areference value from reference circuit 58, the reference value beingindicative of the effective resistance value expected to be obtainedfrom an acceptable coin. The reference value may be stored either as twolimits defining a range, or as a single value to which a tolerance isapplied before comparison. If the comparison indicates acceptability asignal is provided to AND circuit 60.

In practice, one or more other tests will be carried out on the coin,and for each test value that matches a reference value, for the sametype of coin, a further input is applied to AND circuit 60. When all theinputs, one for each of the tests, are present, indicating that the coinbeing tested has produced a complete set of values matching therespective reference values for a given denomination of coin, the ANDcircuit 60 produces an accept signal at its output to cause the coin tobe accepted, for example by operating an accept/reject gate in wellknown manner.

Facilities for carrying out one particular further test, indicative ofthe amount of inductance introduced by the coin into the tuned circuit 2and hence dependent upon a different characteristic or combination ofcharacteristics of the coin than was the resistance test, are alsoincluded in FIG. 1. The value of f0 (i.e. oscillation frequency withoutany imposed phase shift) is applied to a switch 62 via line 64. Switch62 is operated by the arrival sensing and peak detecting section 46 ofcontrol circuit 30 in the same manner as switch 42. Consequently, the"coin absent" or idling frequency without phase delay becomes stored instore 66, and the "coin present" peak low frequency reached withoutphase delay as the coin passes the inductor 4 becomes stored in store68. These frequencies are indicative of the total inductance in thetuned circuit itself, and with the additional influence of the coin,respectively. They are subtracted by a subtracter 70 to give a valueindicative of the inductance change caused by the coin, which iscompared in a comparator 72 with a reference value for an acceptablecoin stored in reference circuit 74, in a similar way to the comparisonmade by comparison circuit 56 as described above. The output ofcomparator 72 forms a further input to AND gate 60 so that the coin canonly be accepted when both the effective resistance and the inductanceit introduces into the tuned circuit 2 are appropriate to the samedenomination of acceptable coin.

The embodiment of FIG. 1 has been described above, and illustrated, interms of switches and functional blocks, but all the components shownwithin the broken-line box 76 can be implemented by means of a suitablyprogrammed microprocessor. The programming falls within the skills of aprogrammer familiar with the art, given the functions to be achieved asexplained above.

FIG. 3 relates to a modification of the apparatus of FIG. 1 whichcompensates for the fact that successive frequency measurements takenwhen the phase shift is in the circuit, and when it is not, relaterespectively to the coin when it is in two different positions, sinceessentially the two frequency measurements are made at different times,and the coin is moving.

FIG. 3 shows a storage array 80 which, in conjunction with a suitablecomputing facility (not shown) is in effect substituted for thecomponents which lie between switch 34 on the one hand, and subtracters54 and 70 on the other hand, in FIG. 1. In the illustration of the array80, the vertical axis represents time. The successive values of f0 areloaded into column A of the array, the values being indicated as A₁ . .. A₃₂. The successive values of fφ are loaded into column B, these beingindicated as B₁ . . . B₃₂. The fφ measurements are interleaved, in time,between the f0 measurements because, of course, it is not possible tomeasure both simultaneously which, with a moving coin, would bedesirable if it were possible.

To compensate for this, compensated values (f'φ) of fφ are calculatedand entered into column C. The first compensated value C₁ is the averageof real values B₁ and B₂, the compensated value C₂ is the average ofreal values B₂ and B₃, and so forth. By this process of interpolation, aset of values for f'φ are developed in column C which, to a reasonableapproximation, are what the corresponding values of fφ would have beenif it had been possible to measure them at the same time as f0 was beingmeasured. Compensated values of Δf can be computed from the f0 values incolumn A and the f'φ values in column C, for example A₂ -C₁ and soforth. Consequently, columns A and D of the array will respectivelycontain the histories of the frequency of oscillation without phaseshift, and the compensated frequency shift caused by the phase shift, asa coin moves past the inductor.

The time at which a coin starts to enter the field of the inductor maybe detected in various known ways, for example by constantly checkingfor f0 changing by more than a predetermined amount in a givenpredetermined short period of time. Such detection can be used to definea position in the array, indicated by broken line 82, above which thevalues relate to the coil alone and below which the values relate to thecoil as progressively influenced by the coin entering into, andeventually moving out of, its field.

A peak value of R for the coin alone can be computed by subtracting fromthe peak value of Δf occurring below line 82 a value of Δf which occursabove line 82. Preferably, though, for additional accuracy, an averageof several Δf values occurring around the maximum value will be taken torepresent the peak, and an average of several Δf values occurring beforethe coin arrives will be taken to represent the idling value. A peakvalue of L for the coin alone may be calculated in similar manner butusing the f0 values from column A of the array.

Alternatively, values of R and L for the coil as influenced (if at all)by a coin may be calculated for each pair of f0 and Δf values occurringin columns A and D, the calculated R and L values being entered incolumns E and F of the array. Columns E and F will then contain thehistories of R and L, for the coil plus any influence of the coin frombefore the coin arrives until after it has left the inductor, thesevalues of course relating to the coil alone during the periods beforearrival of the coin and after its departure. This enables not only peakvalues for R and L of the coin alone, but also non-peak values ifdesired, to be derived, by subtraction, from columns E and Frespectively.

However the values are derived, they may be compared with references asdescribed in relation to FIG. 1.

Although the inductor is shown as a single coil, it may have otherconfigurations, such as a pair of coils opposed across the coinpassageway and connected in parallel, series aiding or series opposing.

In the above description, reference has been made to making measurementswhen the oscillator frequency is at a peak value. However, because thefrequency is measured only at intervals, it is possible, and indeedlikely, that on many occasions the measured values do not include theexact most extreme frequency value that is actually reached, or wouldhave been reached if oscillation frequency had not been altered by theintroduction or removal of phase shift, that is to say the measurementsrelied upon are taken while frequency is changing. It is also known todeliberately make use of measurements which are taken whilst frequencyis changing due to movement of the coin. It is in these circumstancesthat the compensation technique, particular by interpolation, enablesthe greatest improvement in accuracy to be achieved.

It is thought to be desirable, in order for a peak measurement to beadequately representative of the actual extreme value, or extreme valuethat would have occurred, for at least ten R values to be measuredduring the passage of the smallest acceptable coin past the sensor,involving ten measurements with phase shift and ten without. Presently,the smallest of the world's coins needing to be accepted would be theDutch 10 cent coin having a diameter of 15 mm, in which caseapproximately ten R measurements would need to be made per 15 mm of cointravel, the result then being more than ten such measurements when thesame sampling rate was applied to coins of larger diameter. It has beenfound that this can be achieved if the track on which the coin movesfreely is inclined at an angle of between 10° and 20° to the horizontal,preferably between 13° and 15° and the periods for which the phase shiftis switched in, and also out, are respectively not longer than about 1.6mS, and preferably around 0.8 mS.

It is known that measurements taken using relatively high and relativelylow frequencies give information about the coin material at differentdepths within the coin, due to the skin effect. The invention enablesthe effective resistance in the tuned circuit to be measured at higherfrequencies than is practically possible using amplitude-measurementtechniques. Hence, the invention enables effective resistancemeasurements to be made more selectively.

Although in the prior art techniques based on amplitude measurements itwas the intention to determine the effective resistance introduced intoa circuit by the proximity of a coin, it was known that amplitude wassensitive to variations in parameters other than effective resistanceand this was a source of potential error. Hence, it was desirable totake special design steps to minimise or compensate for the variationsin the relevant parameters, and this increased cost and complexity. Thephase-change induced frequency shift used in the present invention issubstantially insensitive to variations in parameters other thaneffective resistance in the tuned circuit, and therefore by subtractingthe "coin absent" measurement from the "coin present" measurement a moreaccurate determination of the effective resistance introduced by thecoin itself can be made, without additional costly steps, including thecost of a coin stopping and releasing mechanism as required by the priorart mentioned previously.

Furthermore, whereas amplitude takes a period of time to stabilise afterthe oscillator is switched on, frequency becomes established at a stablevalue virtually instantaneously, so that the invention facilitatesswitching the sensors in a multi-sensor apparatus on and off one at atime to save power or avoid interference, or both, without resorting toan undesirably slow rate of switching.

I claim:
 1. A method of testing coins using an oscillating tuned circuitwhich includes an inductor, three parameters of the tuned circuit beinginterdependent, namely:a) the effective resistance in the circuit b) thephase of a signal in the circuit, and c) the frequency of oscillation ofthe circuit, the method comprising imposing a change in said phase whena coin is adjacent to the inductor, deriving from the resultingfrequency change a value dependent on the effective resistance in thetuned circuit as influenced by the coin, and using the derived value ina coin acceptability check, characterised by causing the coin to movepast the inductor during said phase change and the resulting frequencychange.
 2. A method as claimed in claim 1 wherein the oscillator is afree-running oscillator having a feedback path, and comprising changingthe phase shift occurring in the feedback path.
 3. A method as claimedin claim 1 comprising imposing said change when there is no coinadjacent to, and also when there is coin adjacent to, said inductor, andderiving said value as a function of both of the "coin present" and"coin absent" changes in frequency.
 4. A method as claimed in claim 3,comprising deriving said value as the difference between the "coinpresent" and "coin absent" changes in frequency.
 5. A method as claimedin claim 1, comprising deriving an inductance-dependent value which is afunction of the frequency when there is no coin adjacent to, and alsowhen there is a coin adjacent to, said inductor when said phase is thesame in both cases, and using the derived inductance-dependent value insaid coin acceptability check.
 6. A method as claimed in claim 1,comprising measuring said frequency with and without said imposed phasechange, and compensating the derived value for the effect of the changein position of the moving coin occurring between the two frequencymeasurements.
 7. A method as claimed in claim 6 comprising repeatedlyimposing, then removing, said phase change, repeatedly measuring saidfrequency with and without the imposed phase change, interpolatingbetween either the frequency values measured with the phase change, orthose measured without the phase change, to develop compensatedfrequency values, and utilising the compensated frequency values inarriving at said frequency change.
 8. Apparatus for testing coins,comprising a tuned circuit including an inductor and means for causingthe tuned circuit to oscillate, three parameters of the tuned circuitbeing interdependent, namely:a) the effective resistance in the circuitb) the phase of a signal in the circuit, and c) the frequency ofoscillation of the circuit, means for positioning a coin adjacent tosaid inductor so as to influence the effective resistance in the tunedcircuit, means for imposing a change in said phase, means for derivingfrom the resulting change in said frequency a value dependent on theeffective resistance in the tuned circuit as influenced by the coin, andmeans for using the derived value in a coin acceptability check,characterised in that the means for positioning the coin is a coinpassageway arranged to permit the coin to move freely past the inductorwhile said phase change is being imposed.
 9. Apparatus as claimed inclaim 8 wherein said means for causing the tuned circuit to oscillate isa feedback path including a gain element, whereby to form with the tunedcircuit a free-running oscillator.
 10. Apparatus as claimed in claim 9comprising phase changing means in the feedback path.
 11. Apparatus asclaimed in claim 8 comprising control means for operating thechange-imposing means when there is no coin adjacent to, and also whenthere is a coin adjacent to, said inductor, and wherein said derivingmeans is operable to derive a value which is a function of the "coinpresent" and "coin absent" changes in frequency.
 12. Apparatus asclaimed in claim 11 wherein said deriving means takes the differencebetween the "coin present" and "coin absent" changes in frequency. 13.Apparatus as claimed in claim 8 including means for sensing saidfrequency, means for deriving from the sensed frequency a valuedependent on the effective inductance in the tuned circuit as influencedby the coin, and means for using the derived inductance-dependent valuein said coin acceptability check.
 14. Apparatus as claimed in claim 13comprising means for detecting the sensed frequency when there is nocoin adjacent to, and also when there is a coin adjacent to, saidinductor when said phase is the same in both cases, and wherein themeans for deriving the inductance-dependent value derives that value asa function of the "coin present" and "coin absent" frequencies. 15.Apparatus as claimed in claim 8 comprising means for measuring saidfrequency with and without said imposed phase change, and means forcompensating the derived value for the effect of the change in positionof the moving coin occurring between the two frequency measurements. 16.Apparatus as claimed in claim 15 wherein said phase change imposingmeans is operable to repeatedly impose, then remove, said phase change,said frequency measuring means is operable to measure said frequencyrepeatedly with and without the imposed phase change, and saidcompensating means develops compensated frequency values from either thefrequency values measured with the phase change, or those measuredwithout the phase change, by interpolating between the measured values,said deriving means being adapted to derive said resistance-dependentvalue, from a frequency change arrived at using the compensatedfrequency values.